The effects structural alterations may have on the carcinogenicity or toxicity of various compounds are determined. Methods to inhibit the action of known carcinogens by administration of other compounds are devised. The biochemical, morphological, and physiological bases for such inhibitory effects are studied. Characterization and isolation of cytochrome P-450 species involved in the metabolism of dimethylnitrosamine (DMN) are under investigation. The importance of the active liver microsomal enzyme system on the tumor production in the peripheral tissues is evaluated.